It is desirable in the reproduction of recorded signals, particularly signals recorded on magnetic media, to compensate for frequency response roll-off. In videotape recording or reproducing this must be corrected before the signal enters the frequency demodulator. Even in an audio system it is desirable and sometimes necessary to correct for the amplitude roll-off with frequency.
In the past, this has been corrected by an equalizer circuit which utilizes a time delay having a plurality of time delay elements connected in series. An improved form of such a circuit is described in my above-identified co-pending patent application.
In situations where the reproduction of the recorded signals involve the use of a plurality of magnetic recording heads, such as the rotating magnetic heads in a videotape recording and reproducing apparatus, the roll-off compensation is different for each magnetic head. Also, it is sometimes desirable to shift the range over which the frequency response is to be compensated to produce a flat response.
While there exist prior art methods which would be suitable for controlling an RF equalizer circuit, these prior art methods cause a control signal to be introduced in the signal being reproduced from the magnetic media. This is obviously undesirable. What is required is a controllable RF equalizer which does not superimpose the equalization control signal upon the output signal. The circuit of my above referenced co-pending application solves this problem, but with greater complexity than that of the present invention.